Burden of elevated lipoprotein(a) among patients with atherosclerotic cardiovascular disease: Evidence from a systematic literature review and feasibility assessment of meta-analysis

Background Elevated lipoprotein(a) [Lp(a)] level is an independent genetic risk factor that increases the risk of atherosclerotic cardiovascular disease (ASCVD) by 2–4 fold. We aimed to report the burden of clinically relevant elevated Lp(a) in secondary prevention ASCVD population as the evaluation of such evidence is lacking. Methods A systematic literature review (SLR) was conducted using Embase®, MEDLINE®, and MEDLINE® In-Process databases to identify studies reporting burden of elevated Lp(a) levels from January 1, 2010, to March 28, 2022. Full-text, English-language studies including ≥500 participants with ≥1 Lp(a) assessment were included. Results Sixty-one studies reported clinical burden of elevated Lp(a). Of these, 25 observational studies and one clinical trial reported clinical burden of clinically relevant elevated Lp(a) levels. Major clinical outcomes included major adverse cardiovascular event (MACE; n = 20), myocardial infarction (MI; n = 11), revascularization (n = 10), stroke (n = 10), cardiovascular (CV) mortality (n = 9), and all-cause mortality (n = 10). Elevated Lp(a) levels significantly increased the risk of MACE (n = 15) and revascularization (n = 8), while they demonstrated a trend for positive association with remaining CV outcomes. Meta-analysis was not feasible for included studies due to heterogeneity in Lp(a) thresholds, outcome definitions, and patient characteristics. Three studies reported humanistic burden. Patients with elevated Lp(a) levels had higher odds of manifesting cognitive impairment (odds ratio [OR] [95% confidence interval; CI]: 1.62 [1.11–2.37]) and disability related to stroke (OR [95% CI]:1.46 [1.23–1.72)]) (n = 2). Elevated Lp(a) levels negatively correlated with health-related quality of life (R = −0.166, p = 0.014) (n = 1). A single study reported no association between elevated Lp(a) levels and economic burden. Conclusions This SLR demonstrated a significant association of elevated Lp(a) levels with major CV outcomes and increased humanistic burden in secondary prevention ASCVD population. These results reinforce the need to quantify and manage Lp(a) for CV risk reduction and to perform further studies to characterize the economic burden.


Introduction
Lipoprotein(a) [Lp(a)] is an atherogenic lipoprotein produced in the liver by the covalent assembly of apolipoprotein B-100 with apolipoprotein A [1]. Elevated Lp(a) levels are found in one in five people worldwide and have been associated with an increased risk of cardiovascular disease (CVD) [2][3][4].Mendelian randomization studies have established Lp(a) as an independent genetic and causal risk factor for myocardial infarction (MI), ischemic stroke, and coronary artery disease (CAD) in individuals with or without CVD [5][6][7].
Atherosclerotic cardiovascular disease (ASCVD) is characterized by atherosclerotic plaques resulting from the deposition of lipids in areas of arteries with disturbed nonlaminar blood flow [7].The 2018 American Cholesterol Clinical Practice Guidelines consider elevated Lp(a) as a risk-enhancing factor for ASCVD [8].Despite the existing evidence on the association of elevated Lp(a) levels with an increased ASCVD risk, Lp(a) profiling in patients has not been adopted in clinical practice.Lp(a) levels are genetically determined and cannot be significantly modified by diet and exercise [1].The 2018 American Cholesterol Clinical Practice Guidelines suggest the initiation of statin therapy for patients with elevated Lp(a) [8].Statins reduce the overall lipid levels, yet they do not have a clinically meaningful impact on Lp(a) levels.Thus, the lack of approved Lp(a)-targeting therapy for lipid management and ASCVD event risk reduction is an unmet need.
Guidelines from the World Health Organization (WHO), International Federation of Clinical Chemistry and Laboratory Medicine (IFCCLM), National Lipid Association (NLA), American College of Cardiology/American Heart Association (ACC/AHA), Canadian Cardiovascular Society (CCS), American Association of Clinical Endocrinologists (AACE), and American College of Endocrinology (ACE) suggest Lp(a) values of �50 mg/dL, �100 nmol/L, or �125 nmol/L as clinically relevant elevated Lp(a) thresholds [9,10].
Chinese guidelines for the management of dyslipidemia have defined elevated Lp(a) levels conferring CV risk as �30 mg/dL [12].The Australian integrated guidance on patients with familial hypercholesterolemia (FH) with ASCVD has defined elevated Lp(a) levels as �150 nmol/L or �70 mg/dL [13,14].In addition, Kamstrup et al. defined the 95 th percentile of Lp(a) or �90 mg/dL as elevated Lp(a) and a risk factor for aortic valve stenosis and ischemic heart disease in the general population [15].
To the best of our knowledge, no studies have systematically evaluated the disease burden associated with guideline-recommended clinically relevant elevated Lp(a) levels in patients with ASCVD.We performed a systematic literature review (SLR) to understand the clinical, humanistic, and economic burden across clinically relevant Lp(a) levels in the ASCVD population.

Materials and methods
The SLR was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [16].The PRISMA checklist is provided as S1 Table .Embase 1 , MEDLINE 1 , and MEDLINE 1 In-Process databases were searched for articles published from January 1, 2010, to March 28, 2022.The MEDLINE 1 Epub Ahead of Print, In-Process, and Other Non-Indexed Citations were searched on PubMed ( March 28, 2022).Searches were limited to English-language articles.A bibliographic search of relevant reviews was performed to identify additional publications.The search strategies along with detailed inclusion and exclusion criteria used to conduct the SLR are provided in S2-S4 Tables.
Briefly, studies were included if they had: (a) patients with established ASCVD, (b) �500 participants, and (c) reported clinical, humanistic, or economic burden associated with elevated Lp(a) in the secondary prevention of ASCVD, regardless of the Lp(a) unit (mg/dL vs nmol/L) used for measurement.Due to paucity of evidence for humanistic and economic burdens, the sample size restriction criteria of �500 patients was not applied for these reviews.Observational studies, randomized controlled trials (RCTs), post hoc analyses, and meta-analyses were included.Titles and abstracts were screened by two independent reviewers.A third independent reviewer resolved discrepancies by consensus.Full-text publications were screened, and those satisfying the inclusion criteria were selected for data extraction.Multiple publications from the same study were linked and extracted as a single study.Data extraction was performed by one reviewer.Data quality checks were performed by the second reviewer and differences were reconciled by the third reviewer.Observational studies and RCTs were critically appraised on the Newcastle-Ottawa Scale (NOS) [17] and Cochrane Collaboration's Risk of Bias Tool, respectively [18].
Clinical burden is summarized as clinical outcomes, including composite major adverse cardiovascular event (MACE), cardiovascular (CV) and all-cause mortality, MI, stroke, and revascularization, reported as events percentage, event rates per 100 patient/person-years, and hazard ratio (HR), odds ratios (OR), or relative risk (RR) with 95% confidence intervals (CIs).
Humanistic burden was assessed differently in all the included studies.Cognitive impairment was defined as Montreal Cognitive Assessment (MoCA) scores of �22 at 1 year.Cognitive improvement was defined as an MoCA increase of �20% or �30%.Functional outcome was assessed with the modified Rankin scale (mRS) at 3 months and 1 year after stroke.The mRS scale ranges from 0 to 6.An mRS score of 0 was defined as no residual stroke symptoms, 5 as severe disability, and 6 as death.Increased disability related to stroke was defined as an mRS score of �3.Humanistic burden was also assessed using the Short Form Health Survey (SF-36) for health-related quality of life (HRQoL).The HRQoL score included both the physical component summary (PCS) and the mental component summary (MCS), with each comprising four parts.Thus, HRQoL assessed eight items, with each item scoring between 0 (worst) and 100 (best).The weighted standard of the eight items was then converted into the total HRQoL score, with an average of 50±10.The outcomes are reported as patient percentage, events percentage, ORs with 95% CIs, and Spearman's multiple linear regression correlation coefficient.
Economic burden was reported as median hospitalization cost (in 10,000 yuan).Healthcare resource utilization was measured in terms of length of hospital stay (in days).
The outcomes included in the feasibility assessment for network meta-analysis were MACE, CV mortality, all-cause mortality, MI, stroke, and revascularization.The outcomes were identified based on the key clinical outcomes for ASCVD.Heterogeneity in terms of elevated Lp(a) thresholds, references for elevated Lp(a) thresholds, patient populations, study characteristics, patient characteristics, risk factors, comorbidities, and outcomes were assessed.

Study characteristics
This SLR included 106 studies (from 111 publications, including three linked studies), of which 61 (including two linked studies) reported the clinical burden of elevated Lp(a) in patients with ASCVD (Fig 1).Twenty-six studies reported clinical burden of clinically relevant elevated Lp(a) in patients with ASCVD.Of the 26 studies, 46% were from Asia (n = 12), 35% were from Europe (n = 9), 8% were from the North America (n = 2), 4% were from Oceania (n = 1), and 8% were multinational (n = 2, including one multinational study from Europe).The study design of the included studies was prospective observational (n = 20; 77%), retrospective observational (n = 5; 19%), and RCTs (n = 1; 4%).The follow-up duration of the studies varied from 1 year to 11 years.The sample size of the studies ranged from 536 to 413,734.One of the 26 studies with a retrospective study design from China included for clinical burden also provided data for economic burden (sample size: 944 and median follow-up: 1.4 years).Additionally, two prospective observational studies with 1 year follow-up duration and a single cross-sectional study from China reported data for humanistic burden (sample size range: 456 to 9709).The characteristics of the included studies are summarized in Table 1.

Patient characteristics
Thirteen of the twenty-six studies reported clinical burden for ASCVD, ten studies reported data for ACS, and one study each reported data for peripheral artery disease (PAD), stroke, and revascularization.Fifteen studies defined elevated Lp(a) levels as �30 mg/dL, four as �50 mg/dL, and one as �70 mg/dL.No studies were available for elevated Lp(a) defined as �90 or �180 mg/dL or super-elevated Lp(a) defined as �300 mg/dL.One study defined elevated Lp (a) as >75 nmol/L, while two studies defined elevated Lp(a) level as �100/�125/�150 nmol/L and super-elevated Lp(a) level as �200 nmol/L.No studies were available for elevated Lp(a) defined as �90 nmol/L or super-elevated Lp(a) defined as �400/�430 nmol/L.Seventeen of the twenty-six studies (including one study that reported data for economic burden) reported patient characteristics for patients with elevated Lp(a) levels.In these studies, the mean age ranged from 56.6 to 71.3 years.All studies reported a higher proportion of male patients (53%-79%) and patients with hypertension (54%-89%).All studies reported a lower proportion of patients with diabetes mellitus (3.5%-35%).Only four studies reported varying proportion of patients with dyslipidemia (12%-58.8%).Five studies reported the proportion of patients with a family history of CVD (15%-68%).High variability was observed in the measurement units for CV biochemical markers, including different lipids, hemoglobin A1c (HbA1c), and high-sensitivity C-reactive protein (hsCRP).Nine of the twenty-six studies included for clinical burden did not report patient characteristics [19,22,27,[35][36][37][38]40,42].
In a single study that reported the economic burden and clinical burden, the mean age of ACS patients with elevated Lp(a) levels (�30 mg/dL) was 65.5 years.The study had a higher proportion of male patients (77%) and patients with hypertension (62%) [43].
Two of the three studies contributing to humanistic burden reported patient characteristics; however, one study, where Lp(a) was reported as a continuous variable, did not report patient characteristics [44].In the former studies, stroke patients with elevated Lp(a) levels (defined as �35.8 mg/dL by Li et al. [46] and as quartile 4 by Jiang et al. [45]) had a higher proportion of male patients (68%-77%) and patients with hypertension (63%-78%) [45,46].The median age  $ In Li 2021 [46], the Lp(a) threshold for quartile 4 (Q4) is not mentioned.~Puri 2017 [35] involves multiple countries, including the USA, Canada, and Australia.

±
Bao 2021 [44] was included for providing evidence for humanistic burden despite having a sample size of <500, due to paucity of evidence.¥ Lp(a) was measured in 226 male premature chronic heart disease (PCHD) patients.The sample size of patients with elevated Lp(a) was not reported.€ Lp(a) was considered as a continuous variable and the thresholds for elevated Lp(a) were not reported.https://doi.org/10.1371/journal.pone.0294250.t001 of the patients ranged from 61 to 63 years.The characteristics of the included studies are summarized in Table 2.

Clinical burden
MACE.MACE as the clinical outcome in patients with ASCVD was reported in 20 studies.The definition of MACE varied across the included studies.Ten studies were from Asia, six were from Europe, two were from North America, one was from Oceania, and one was a multinational study.Nineteen studies reported HRs, three studies reported the events percentage of MACE, two studies reported the event rates for MACE, one study reported the OR, and one study reported the RR.The risk of MACE associated with elevated vs lower Lp(a) levels was shown to be significantly greater in 14 of the 16 studies reporting HRs from multivariate analyses.The HRs in these studies ranged from 1.14 (95% CI, 1.02-1.29)to 32.92 (95% CI, 21.35-50.77)[20][21][22]25,27,28,31,32,34,36,37,40,41,43].Three of the 16 studies reporting HRs from multivariate analyses showed a trend for association between elevated Lp(a) levels and MACE vs low Lp(a) levels; however, the association was not significant (Fig 2A) [22,24,30].Twelve studies reported HRs from univariate analyses (Fig 2B).Eight of 12 studies reported a positive association between elevated Lp(a) levels and MACE vs low Lp(a) levels (HR [95% CI] range: 1.22 [1.12-1.32] to 1.86 [1.38-2.49])[20,21,25,27,28,32,41,42]. Of the remaining four studies, one study showed positive association for MACE in the Cox proportional model, while it showed a trend for association in the log-rank test [43].Two studies showed a trend for association between elevated Lp(a) levels and MACE (however, the results were not significant) [24,39], while the other study showed no association between the variable and the outcome [30].The single RCT also reported a trend for association between elevated Lp(a) levels (�50 mg/dL) and MACE vs low Lp(a) levels (<50 mg/dL); however, the association was not significant.For this study, the analysis type (i.e., univariate or multivariate) was not reported [35] (Fig 2B).
CV mortality and all-cause mortality.Nine studies provided data for the outcome of CV mortality in patients with ASCVD.Six studies were from Asia, two were from Europe, and one was from Oceania.While eight studies reported the events percentage of CV mortality, six studies reported the HRs, and one study reported the OR.All six studies reporting HR defined elevated Lp(a) as �30 mg/dL.In three of the six studies providing HRs from multivariate analyses, elevated Lp(a) levels were significantly associated with an increased risk of CV mortality, with HRs ranging from 1.519 (95% CI, 1.083-2.132)to 1.9 (95% CI, 1.1-3.4)vs low Lp(a) levels   [32] reported four definitions for MACE and for each definition, results of univariate and multivariate analysis are reported: *CV death with spontaneous MI, ischemic stroke, and any repeat revascularization; ^The composite of CV death and spontaneous MI; # The composite of CV death, excluding unknown cause of death, spontaneous MI, and ischemic stroke; and @ Primary outcomes: the composite of CV death, spontaneous MI, and ischemic stroke.Welsh 2020 [40] reported three definitions for MACE outcomes: & Coronary heart disease, ~Fatal CVD, and $ Composite CVD outcome.₶ Wang 2020 [37] defined MACE as cardiac death and ACS, including unstable angina pectoris and acute MI (AMI).± Wohlfahrt 2021 [39] defined MACE as recurrent cardiovascular events (CVE), including hospitalization for ACS, ACS recurrence, or death from CV causes.Wong 2021 [36] reported two definitions for MACE outcomes: R Total ASCVD event and § First ASCVD event.
¥ Analysis was done using Cox proportional hazards model.¶ Analysis was done using log-rank test.Ø Represents hard chronic heart disease (CHD) events.Sang 2021 [22] reported two definitions for MACE outcomes: represents the MACE outcomes and Ø represents hard CHD events.Puri 2017 [35] reported two definitions for MACE outcomes: ¤ defined as changes in MACE, death, non-fatal MI, stroke, coronary revascularization, or hospitalization for unstable angina and ‡ defined as death, non-fatal MI, stroke, coronary revascularization, or hospitalization for unstable angina.https://doi.org/10.1371/journal.pone.0294250.g002[22,25,26].Of the three remaining studies, two showed a trend for positive association between elevated Lp(a) levels and CV mortality (however, the results were not significant) [28,32], while one study showed no association [30].Four studies reported the HRs from univariate analyses.One of these four studies reported a significant positive association between CV mortality and elevated Lp(a) levels vs low Lp(a) levels (HR [95% CI] range: 1.29 [1.11-1.49] to 1.47 [1.15-1.89])[32], while two studies showed a trend for positive association (however, the results were not significant) [25,28].The remaining one study reported no association between elevated Lp(a) and CV mortality [30] (Fig 3).The single study reporting OR demonstrated no association between elevated Lp(a) (�30 mg/dL) and CV mortality in patients with ACS vs patients with low Lp(a) levels (<30 mg/dL) [23].Eight studies reported the events percentage of CV mortality.One of the eight studies reported significantly higher CV mortality in patients with acute MI with elevated Lp(a) levels (>30 mg/dL) vs patients with low Lp(a) levels (�10 mg/dL) (31.8% vs 22.8%) [26].In seven of the eight studies, no trend or association was reported [23][24][25]28,30,32,34].
All-cause mortality in patients with ASCVD and elevated Lp(a) were reported in ten studies (including five studies from Asia, three from Europe, one from North America, and one from Oceania).Of the ten studies, seven reported HRs and seven reported the events percentage.Five of the seven studies reporting HRs from multivariate analyses showed a trend for positive association between elevated Lp(a) levels and all-cause mortality (however, the association was not significant) [22,24,32,39,43], while two studies did not show an association [28,30].One of the six studies reporting HRs from univariate analyses showed a significant association between elevated Lp(a) levels (>125 nmol/L vs 10-30 nmol/L) and all-cause mortality (HR [95% CI]: 2.53 [1.03-6.2])[39].Three of the six studies reporting HRs from univariate analyses showed a trend for positive association between elevated Lp(a) levels and all-cause mortality (however, the association was not significant) [24,32,43], while two studies did not show an association [28,30]

(Fig 4).
One of the seven studies reporting all-cause mortality events percentage reported significantly higher events percentage in patients with MI with nonobstructive coronary arteries (MINOCA) and elevated Lp(a) levels (�30 mg/dL) vs patients with MINOCA and low Lp(a) levels (<10 mg/dL) during the 3-year follow-up (3.1% vs 0.5%) [20].Additionally, three of the seven studies reported numerically higher all-cause mortality events percentage in patients with elevated Lp(a) levels; however, the results did not reach statistical significance [31,34,43].The three remaining studies reported inconclusive results [24,28,30].
MI. Eleven studies reported MI as an outcome in patients with ASCVD.Six studies were from Asia, four were from Europe, and one was from North America.Nine studies reported the HRs, two reported the rates of MI, and two reported the ORs.Compared with patients with lower Lp(a) levels, patients with elevated Lp(a) levels had a greater risk of MI in the seven studies reporting HRs from multivariate analyses.Of these seven studies, one study reported a significant association (HR [95% CI]: 23.22 [12.17-44.29])[34], while the remaining six showed a trend for association between elevated Lp(a) levels and MI, although the results did not reach statistical significance [14,25,28,30,32,43].Five studies reported HRs from univariate analyses.One of the five studies showed a significant association between elevated Lp(a) levels (�30 mg/dL) and MI vs low Lp(a) levels (<30 mg/dL) [32], while, three studies showed a trend for positive association (however, the results were not significant) [25,28,43].The remaining one study showed no association between Lp(a) levels and MI [30].Additionally, two studies for which the analysis type was not reported showed a significant association between elevated Lp(a) levels and an increased risk of MI (HR [95% CI] range: 1.54 [1.05-2.27] to 3.56 [1.78-7.1])[19,36] ( Fig 5).Two studies reported the OR for the association between elevated Lp(a) levels and MI.One of these studies reported a 52.1% increased odds of MI with elevated Lp(a) levels (�30 mg/dL) vs low Lp(a) levels (<30 mg/dL) (OR [95% CI]: 1.521 [1.179-1.963][29], while one study showed a trend for association (however, the association was not significant) [23].One of the two studies reporting the event rates for MI reported a numerically higher rate of MI in patients with symptomatic artery disease and elevated Lp(a) (�50 mg/dL; 4.68 per 100 patient-years) vs patients with low Lp(a) levels (<30 mg/dL; 1.36  per 100 patient-years; p value was not reported) [34].The remaining one study reported similar event rates of MI for patients with elevated and low Lp(a) levels [32].
Stroke.The association of Lp(a) with stroke in patients with ASCVD was reported in ten studies.Five studies were from Asia and five were from Europe (including one multinational study from Europe).Of these ten studies, nine reported HRs, two reported rates of stroke, and one reported OR.Three of the nine studies reporting HRs from multivariate analyses showed significant association of elevated Lp(a) levels with stroke vs lower Lp(a) levels (HR [95% CI] range: 1.16 (1.01-1.34] to 64.52 [29.13-142.93])[25,34,40].In most of the remaining studies, a trend for increasing risk of stroke was observed with elevated Lp(a) levels; however, the association was not significant [28,30,32,38,40].Two studies did not report significant results or show an increasing trend for association [14,43].Six studies reported the HRs from univariate analyses.Two of the six studies reported a significant association between elevated Lp(a) levels and an increased risk of stroke (HR [95% CI] range: 1.36 [1.08-1.71] to 1.6 [1.1-2.2])[25,32].Three of the remaining studies showed a trend for association between elevated Lp(a) levels and stroke (however, the results did not reach statistical significance) [28,30,38], while one showed no association [43] (Fig 6).The single study reporting OR showed increased odds of stroke in patients with elevated Lp(a) levels (�30 mg/dL) vs patients with low Lp(a) levels (<30 mg/dL); however, the association was not significant [23].One of the two studies reporting the event rates for stroke reported a numerically higher rate of stroke in patients with symptomatic artery disease and elevated Lp(a) levels (�50 mg/dL; 6.88 per 100 patient-years) vs patients with low Lp(a) levels (<30 mg/dL; 0.89 per 100 patient-years; p value was not reported) [34].The remaining one study reported similar event rates of stroke for patients with elevated and low Lp(a) levels [32].
Revascularization.Ten studies reported revascularization outcome in patients with ASCVD (including four studies from Europe, three from Asia, two from North America, and one from Australia).While all studies reported the HRs, two reported events percentage and one reported the rates of revascularization.Seven of the nine studies reporting HRs from multivariate analyses showed significantly higher risk of revascularization among patients with elevated Lp(a) levels, with HRs ranging between 1.13 (95% CI, 1.02-1.25)and 4.387 (95% CI, 2.052-9.382)[14,24,[31][32][33]40,43]. The two remaining studies showed a trend for association; however, the association was not significant [28,30].Three of the five studies reporting HRs from univariate analyses showed a significant positive association between elevated Lp(a) levels (�30 mg/dL) and an increased risk of revascularization (HR [95% CI] range: 1.15 [1.04-1.27] to 3.765 [1.896-7.476])vs low Lp(a) levels (<30 mg/dL) [24,32,43].Two of the five studies showed a trend for association between elevated Lp(a) levels and revascularization; however, the association was not significant [28,30].Additionally, one study with unknown analysis type reported a significant positive association between elevated Lp(a) levels (�70 mg/dL) and an increased risk of revascularization (HR [95% CI] range: 1.6 [1.13-2.28] to 1.69 [1.17-2.45])vs low Lp(a) levels (�15 mg/dL) [36] ( Fig 7).Two studies reporting results for revascularization events percentage showed numerically higher events percentage for patients with elevated Lp(a) levels (8.9-30.9%)vs patients with low Lp(a) levels (3%-26%; p values were not reported) [31,43].A single study reported similar event rates (per 100 person-years) of revascularization for patients with elevated and low Lp(a) levels [32].

Humanistic burden
Three studies reported data for humanistic burden.In a prospective cohort study from China, patients with acute ischemic stroke (AIS) or TIA from the Third China National Stroke Registry (CNSR-III) with the highest Lp(a) quartile had higher events percentage of cognitive impairment (defined as MoCA �22) at 1 year vs those with the lowest quartile of Lp(a) (35.7% vs 25.5%; p value was not reported).Patients in the highest Lp(a) quartile were at significantly higher odds of cognitive impairment vs patients in the lowest Lp(a) quartile (unadjusted OR [95% CI]: 1.62 [1.11-2.37].Results from multivariate analyses showed similar trend; however, the results were not statistically significant.In contrast, no association was found between Lp (a) levels and cognitive improvement (MoCA increase by �20% or �30%) from 2 weeks to 1 year from discharge after AIS or TIA [46].Another study on patients with AIS or TIA from  [28] reported two definitions for revascularization: # Unplanned target vessel revascularization was defined as any repeat PCI or coronary artery bypass grafting (CABG) and @ Patients with stent stenosis.Golledge 2020 [24] reported two definitions for revascularization: $ Patients undergoing lower limb peripheral revascularization (open or endovascular) and ~Any PAD operation, including the lower limb (open and endovascular) peripheral revascularization, carotid artery revascularization, open and endovascular abdominal aortic aneurysm repair, and other aneurysm repair.*Analysis was done using Cox proportional hazards model.^Analysis was done using log-rank test.Wong 2021 [36] reported two definitions for revascularization: ¤ Cerebral revascularization and ‡ Total coronary revascularization.https://doi.org/10.1371/journal.pone.0294250.g007the CNSR-III reported that elevated Lp(a) levels were significantly associated with increased disability related to stroke (evaluated by an mRS score �3) at 3 months and 1 year (OR [95% CI] for quartile 4 [>35.8mg/dL] vs quartile 1 [<8.9 mg/dL]:1.58[1.34-1.86]and 1.46 [1.23-1.72],respectively) [45].In a cross-sectional study from China that included 226 male patients with PCHD, Spearman's multiple linear regression analysis showed that the MCS score negatively correlated with Lp(a) levels after adjusting for risk factors (R = −0.295,p<0.001).The PCS score did not correlate with Lp(a) levels (R = 0.069, p = 0.314).Lp(a) levels weakly correlated with HRQoL.With an increase in the Lp(a) levels, the HRQoL, as assessed by SF-36, decreased (R = −0.166,p = 0.014) [44].

Economic burden
A retrospective observational study from China that included patients with ACS reported no difference in median length of hospital stay in days for patients with elevated Lp(a) levels (�30 mg/ dL) and those with low Lp(a) levels (<30 mg/dL).The study also reported slightly higher median hospitalization costs (in 10,000 yuan) for patients with elevated Lp(a) levels (�30 mg/dL) vs patients with low Lp(a) levels (<30 mg/dL) (3.83 vs 3.52; the result was not significant) [43].

Feasibility assessment for meta-analysis
The meta-analysis feasibility assessment was done for 61 studies that provided data for clinical burden.Five RCTs or post hoc analysis of RCTs contributed data on clinical burden.Of these five studies, a single RCT provided clinical burden data of clinically relevant elevated Lp(a) thresholds (�50 mg/dL) [35] and the remaining four RCTs or post hoc analysis defined elevated Lp(a) as �59.6 mg/dl [47], >73.7 mg/dL [48], �29.4 nmol/L [49] and >165 nmol/L [50].The assessment of five RCTs or post hoc analysis of RCTs revealed heterogeneity in terms of patient population (i.e., overall ASCVD population vs the subgroup of ASCVD population with specific LDL-C and HDL-C levels and on specific treatments) [47]; elevated Lp(a) and reference or low Lp(a) thresholds; comorbidities; biomarkers, including HDL-C, LDL-C, TC, HbA1C, and hsCRP; gender distribution; and risk factors for ASCVD (smoking history, history of CAD, MI, stroke, and PAD events) [35,[48][49][50].MACE was the only outcome that was commonly reported across the trials.However, the definition of MACE varied widely between the five studies, hence, pooling of results for this outcome was not feasible.
Of the 56 observational studies, only the studies that reported clinically relevant elevated Lp (a) thresholds were considered for meta-analysis.Twelve of the 56 studies reporting clinical burden for elevated Lp(a) levels defined as �30 mg/dL were analyzed for the feasibility of meta-analysis.Of these 12 studies, seven were ruled out for meta-analysis due to variability in patient population and reference Lp(a) thresholds [20,21,24,25,27,31,32].Six studies were excluded for meta-analysis due to variability in definition of outcomes, including stroke [28,30,43], MI [19,28,30,43], revascularization [28,30,43], and MACE [20,22,28,30,43].Finally, three studies reporting CV mortality for patients with elevated Lp(a) levels �30 mg/dL (vs <30 mg/dL) were ruled out for meta-analysis as one study did not provide the sample size for Lp(a) categories as well as baseline characteristics and the two remaining studies were heterogeneous for patient characteristics [22,28,30].
Five of 56 studies reporting clinical burden for elevated Lp(a) levels defined as �50 mg/dL were evaluated for the feasibility of meta-analysis.The assessment revealed heterogeneity in the patient population [51]; reference or low Lp(a) thresholds; comorbidities; biomarkers; gender distribution, and risk factors for ASCVD [32][33][34]41].For these observational studies, as with RCTs and post hoc analysis, the results for MACE (the only commonly reported outcome) could not be pooled due to varying definitions.[7].This SLR provides a holistic overview of current evidence on the clinical burden associated with clinically relevant elevated Lp(a) levels in patients with ASCVD in the secondary prevention setting.Moreover, the SLR showcases the available evidence for humanistic and economic burden of elevated Lp(a) levels in patients with ASCVD.The evidence shows significant association between elevated Lp(a) levels and an increased risk of MACE as well as revascularization.Most of the studies reporting CV mortality, all-cause mortality, MI, and stroke showed significant association or trend for association between elevated Lp(a) levels and the respective outcomes.The studies reporting humanistic burden reported higher cognitive impairment, increased disability related to stroke, and reduced quality of life in patients with elevated Lp(a) levels [44][45][46].The single study included for economic burden reported inconclusive results for association between economic burden and elevated Lp(a) levels [43].
The most reported elevated Lp(a) threshold, defined as Lp(a) level �30 mg/dL (recommended by Chinese guidelines for the management of CV risk in adults with dyslipidemia [12] and EAS [11]), was found to be significantly associated with MACE [20][21][22]25,27,28,31,43] and revascularization [31,32,43].For the remaining CV outcomes, including CV mortality, all-cause mortality, MI, and stroke, most of the included studies reported a trend for association between elevated Lp(a) levels and CV outcomes.
All except one study included for elevated Lp(a) levels defined as �50 mg/dL (as recommended by global WHO and IFCCLM guidelines; American NLA and AHA/ACC guidelines, and CCS guidelines [9]) reported significant association of elevated Lp(a) levels with MACE [32,34], MI [34], stroke [34], and revascularization [33].The single RCT included for this threshold showed a trend for association between elevated Lp(a) (�50 mg/dL) and an increased risk of MACE; however the association was not significant [35].
For elevated Lp(a) levels defined as �70 mg/dL or 150 nmol/L (recommended by the Australian integrated guidance for patients with FH with progressive clinical ASCVD [13]), most of the included studies reported a significant positive association of elevated Lp(a) levels with MACE [36,40], MI [36] and revascularization [14,36,40].Of these studies, a single study reported data for �70 mg/dL [36], while two studies reported data for 150 nmol/L [14,40].
The single study that defined elevated Lp(a) levels as �75 nmol/L (recommended by EAS [11]) showed a significant association of elevated Lp(a) levels with MACE [37].
The two studies included for elevated Lp(a) levels defined as �100 nmol/L (recommended by WHO, IFCCLM, and NLA guidelines [9]) reported significant association between elevated Lp(a) levels and revascularization as well as stroke [40]; however, they showed a trend for association with MACE [39] and all-cause mortality [39,40].
The 2019 Heart UK guidelines defined Lp(a) levels �200 nmol/L as a high risk factor for CVD [9].Both studies included for this threshold found a significant association of elevated Lp(a) levels with an increased risk of MACE [41,42].
Notably, the CV risk associated with Lp(a) remains irrespective of LDL-C levels [6].The Copenhagen General Population Study reported that the incidence rate of MACE associated with elevated Lp(a) levels (>50 mg/mL) was maintained despite varying LDL-C levels [41].In this SLR we identified a study that reported insights from the FOURIER trial focused on patients with CVD who were given the LDL-lowering therapy, evolocumab.The study reported that patients who achieved Lp(a) >29 nmol/L had significantly higher residual CV event rates at 3 years despite achieving LDL-C levels <70 mg/dL compared with patients who had achieved Lp(a) � 29 nmol/L [50].In corroboration, three prospective cohort studies (two from China [20,52] and one from North America [36]) reported significant association of elevated Lp(a) levels with an increased risk of MACE (HR [95% CI] range: 1.26 [1.07-1.48] to 1.59 [1.03-3.13])and CV mortality (HR [95% CI]: 1.31 [1.08-1.6]) in patients with ASCVD with low LDL-C levels (vs low Lp(a) levels).Currently, therapies that target high LDL-C levels and lower the risk of ASCVD are available.However, there are no approved therapies that lower the Lp(a) levels and thereby lower the residual CV risk.Progressive CVD with Lp(a)hyperlipoproteinemia has been approved as separate indication in the guidelines of statutory health insurance funds in Germany for apheresis [53].Furthermore, Australian integrated guidance on enhancing care of patients with FH with ASCVD and elevated Lp(a) levels (�150 nmol/L) has recommended apheresis to reduce ASCVD progression in patients who cannot achieve LDL-C targets despite maximally tolerated drug therapy [13].Apheresis, an extracorporeal process of separating blood components, has been shown to reduce apolipoprotein-Bcontaining LDL-C and Lp(a).Moreover, it is hypothesized that the removal of blood factors.including fibrinogen, coagulation factors, thrombogenic factors, complement factors, inflammatory factors, and adhesion molecules, may reduce coagulation and improve endothelial function.Even though the efficacy of apheresis is unquestionable, apheresis might mediate its treatment effects through reductions in LDL-C or Lp(a) levels or by improvement in endothelium-dependent vasodilation [54,55].Additionally, this invasive procedure is associated with several adverse events, including the risk of venipuncture complications, hypotensive episodes, and excessive bleeding [56].Studies have shown that lowering Lp(a) by 50 mg/dL in the secondary prevention setting may reduce MACE risk by 22% over a short-term period and 45% over lifetime [41,57,58].Thus, effective Lp(a)-targeting therapies for reducing the residual risk of secondary CV events remains an unmet need.
This SLR provides a systematic assessment of clinical burden associated with clinically relevant elevated Lp(a) levels in populations with secondary prevention of ASCVD.Furthermore, it also captures evidence for humanistic and economic burden of elevated Lp(a) levels in patients with ASCVD.This SLR is not restricted to any geography or study design and therefore provides a holistic overview of the burden of elevated Lp(a).Nonetheless, this SLR has some limitations, such as including only English-language studies.None of the included studies reported the association between Lp(a) levels and CV outcomes in different ethnic subpopulations.A study by Brandt et al. on a nationally representative cohort from the United States of America (USA) in the primary prevention setting revealed that Mexican-Americans with elevated (50 mg/dL) Lp(a) levels had the highest risk of MI compared with non-Hispanic Whites and non-Hispanic Blacks [59].Another large case-control study in the primary prevention setting from the USA reported that the risk of stroke significantly increased with elevated Lp(a) levels in African Americans but not in Whites and Hispanics [60].Given the ethnicitydriven impact of Lp(a) on CVD outcomes in the primary prevention setting, lack of similar studies in the secondary prevention setting is another gap identified through this SLR.
The clinical association of Lp(a) with outcomes did not vary much across geographies as exemplified by MACE and revascularization, which were found to be significantly associated with elevated Lp(a) for all the geographies.Studies reporting the influence of gender on the interaction between Lp(a) levels and CV outcomes were not available in the secondary prevention setting.However, a few studies in the primary prevention setting have reported that the association between elevated Lp(a) levels and the risk of MI or stroke is gender sensitive.For both outcomes, the risk of CV outcome increased with elevated Lp(a) levels in male but not in female patients [59,60].Lp(a) levels have been shown to be responsive to estrogen levels, which wane in elderly women during and after menopause.However, the menopausal state of women was not reported in these studies [59].Very few studies (N = 4) reported the subgroup data for different age groups.None of these studies could establish the impact of age on CV outcomes in patients with elevated Lp(a) levels [20,24,32,38].These findings warrant further research to understand the underlying effects of gender and age on Lp(a) levels and associated CV risk in the secondary prevention setting.
Patients in the included studies had different ASCVD conditions and different comorbidities, which may confound the results if there is lack of adjustment [61].The varied thresholds determining the exposed group (with elevated Lp(a) levels) and unexposed group (with low Lp (a) levels) in the included studies is a major limitation of this SLR.The skewness in Lp(a) thresholds might have implications on the sample size in exposed and unexposed groups.This would lead to the comparison between elevated and low Lp(a) populations with unmatched sample size.The variability in the methods for Lp(a) measurements (immunoturbidimetry, enzyme-linked immunosorbent assay, or immunonephelometry), time of Lp(a) measurement, and processing of samples may introduce confounders and enrich the underestimated or overestimated Lp(a) values across the subgroups [62].Nearly three-fourths of the included studies in this SLR either reported crude estimates or estimates adjusted for common covariates, including age and gender.Few studies adjusted for comorbidities, smoking status, family history, lipid profile, and lipid-lowering medications.These limitations warrant appropriately designing real-world evidence studies by adopting adequate methodologies for statistical adjustments for critical covariates and definitions of elevated and low Lp(a) levels to illustrate the impact of this causal CV risk factor on ASCVD outcomes.
A major limitation of this SLR is that we synthesized and presented the findings qualitatively as a meta-analysis was not feasible for the included evidence due to heterogeneity in patient population; reference thresholds (or low Lp(a) levels); comorbidities; biomarkers; gender distribution, risk factors for ASCVD; and definition of outcomes.Finally, limited studies evaluating the economic and humanistic burden of elevated Lp(a) was another key gap identified from this SLR.

Conclusions
The findings of this SLR indicate that patients with ASCVD with elevated Lp(a) are at an increased risk of MACE, CV mortality, MI, stroke, revascularization, cognitive impairment, disability, and compromised quality of life.The evidence suggests that Lp(a) is an important biomarker that should be tested as part of routine lipid screening and ASCVD risk profiling.Absence of approved treatments for the secondary prevention of ASCVD in patients with elevated Lp(a) levels mandates the ongoing clinical research to provide therapies that reduce the CV risk associated with elevated Lp(a) levels.Our SLR warrants further high-quality studies in different geographies and ethnic groups to characterize the implications of elevated Lp(a) on clinical burden.Moreover, new studies assessing the economic and humanistic burden of ASCVD are needed to fill the lacunae caused by the paucity of evidence in this domain.

Fig 1 .
Fig 1. PRISMA flow diagram.PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analyses.*Includes three linked studies.$ These objectives are reported in overlapping studies.# Exclusion criteria of sample size <500 was applied for the clinical burden objective only.https://doi.org/10.1371/journal.pone.0294250.g001

Fig 2 .
Fig 2. HR (95% CI) for the association of Lp(a) and MACE. A. Results of multivariate analysis and B. Results of univariate analysis or from studies where analysis type was not reported.ACS: Acute coronary syndrome; ASCVD: Atherosclerotic cardiovascular disease; CI: Confidence interval; CV: Cardiovascular; CVD: Cardiovascular disease; HR: Hazard ratio; Lp(a): Lipoprotein(a); MACE: Major adverse cardiac event; MI: Myocardial infarction; NR: Not reported; PAD: Peripheral artery disease; Revasc.: Revascularization; UK: United Kingdom; USA: United States of America.Yoon 2021[32] reported four definitions for MACE and for each definition, results of univariate and multivariate analysis are reported: *CV death with spontaneous MI, ischemic stroke, and any repeat revascularization; ^The composite of CV death and spontaneous MI; # The composite of CV death, excluding unknown cause of death, spontaneous MI, and ischemic stroke; and @ Primary outcomes: the composite of CV death, spontaneous MI, and ischemic stroke.Welsh 2020[40] reported three definitions for MACE outcomes: & Coronary heart disease, ~Fatal CVD, and $ Composite CVD outcome.₶ Wang 2020[37] defined MACE as cardiac death and ACS, including unstable angina pectoris and acute MI (AMI).± Wohlfahrt 2021[39] defined MACE as recurrent cardiovascular events (CVE), including hospitalization for ACS, ACS recurrence, or death from CV causes.Wong 2021[36] reported two definitions for MACE outcomes:

Fig 7 .
Fig 7. HR (95% CI) for the association of Lp(a) and revascularization.ACS: Acute coronary syndrome; ASCVD: Atherosclerotic cardiovascular disease; CI: Confidence interval; HR: Hazard ratio; Lp(a): Lipoprotein(a); NR: Not reported; PAD: Peripheral artery disease; PCI: Percutaneous coronary intervention; Revasc.: Revascularization; UK: United Kingdom; USA: United States of America.Zhu 2020[28] reported two definitions for revascularization: # Unplanned target vessel revascularization was defined as any repeat PCI or coronary artery bypass grafting (CABG) and @ Patients with stent stenosis.Golledge 2020[24] reported two definitions for revascularization: $ Patients undergoing lower limb peripheral revascularization (open or endovascular) and ~Any PAD operation, including the lower limb (open and endovascular) peripheral revascularization, carotid artery revascularization, open and endovascular abdominal aortic aneurysm repair, and other aneurysm repair.*Analysis was done using Cox proportional hazards model.^Analysis was done using log-rank test.Wong 2021[36] reported two definitions for revascularization: ¤ Cerebral revascularization and ‡ Total coronary revascularization.

Table 1 . Study characteristics of studies included for clinical, humanistic, and economic burden among patients with ASCVD and elevated Lp(a). Study name Region Country Study design Median follow-up duration (years) Population N (study sample size) n (sample size for elevated Lp(a)) Lp(a) threshold (mg/dL) Clinical burden
ACS: Acute coronary syndrome; ASCVD: Atherosclerotic cardiovascular disease; CSS: Cross-sectional study; Lp(a): Lipoprotein(a); NR: Not reported; PAD: Peripheral artery disease; PCS: Prospective cohort study; RCS: Retrospective cohort study; RCT: Randomized controlled trial; UK: United Kingdom; USA: United States of America.*Represents mean values # represents average values, and ^represents mean or median not mentioned/reported; @ represents nmol/L.